Generally, various copper based alloys are selected for use as a bearing material according to conditions such as oil lubricating conditions, sliding speed and sliding contact surface pressure. For bearing materials used in oil, comparatively soft bronze (e.g., BC3 and BC6), phosphor bronze (e.g., PBC2A), lead bronze (e.g., LBC 2-5) and kelmet (e.g., KJ 1-4) casting materials are utilized. In conditions a little poor in oil lubricity, Cu—Sn or Cu—Sn—Pb is used as a copper based sintered bearing material and bronzed based oil-less bearings produced by adding graphite (solid lubricant) to Cu—Sn or Cu—Sn—Pb (copper based sintered bearing material) are often used.
In the roller sections of undercarriages for construction machines, double-layered bearings are utilized which are produced by scattering a lead bronze based sintered material powder onto a metal backing, followed by sintering and rolling with a mill and by sinter bonding the sintered material to the metal backing by resintering. Such double-layered bearings to which a soft metal such as Sn is overlaid are widely used as an engine metal. Under sliding conditions with higher surface pressure, slower sliding speed and a likelihood of boundary lubrication, soft high strength brass (e.g., HBsC 1-4) having good seizure resistance and wear resistance is utilized (see “Engineering Data Book for Copper based Alloy Casting” (pp 134-155), edited by Japan Non-ferrous Metal Casting Association, issued by Materials Process Technology Center (SOKEIZAI CENTER), Jul. 30, 1988).
The recent demand for the most generally used bronze based and lead bronze based contact materials is to achieve improved seizure resistance and wear resistance under high sliding speed conditions while ensuring excellent wear resistance under slow sliding speed and poor lubricating conditions. In view of the current environmental problems, it is desirable to exert, without use of Pb, the characteristics of lead bronze sintered contact materials, namely, good comformability and constant seizure resistance.
It is conceivable that the frequent occurrence of galling accompanied by abnormal abrasion under high speed and high surface pressure sliding conditions is attributable to occurrence of agglutination/adhesion and its rapid growth caused by a contact between the metals in boundary lubrication. In many cases, great effort is made to restrain the galling by forming an overlaid layer from a soft metal (e.g., Sn) like the case of engine metals, thereby achieving improved comformability and fluid lubricity. However, this technique has revealed a problem in the durability and service life of the overlaid layer when the lead bronze based sintered contact material is used under higher surface pressure or in a condition where boundary lubricity increases because of additional conditions (e.g., vibration load and acceleration/deceleration) imposed on the material while the material being subjected to sliding contact. Therefore, it has become necessary to improve the sliding performance and durability of the lead bronze based sintered contact material itself.
On the other hand, lead bronze based and lead copper based sintered contact materials which contain large amounts of Pb suffer from the following problem: If they are used in a condition where, among others, sliding speed is high, or where acceleration and deceleration are repeated with changes in the rotating (sliding) direction so that sliding speed greatly changes, or where the mating material has high surface roughness, wear resistance will rapidly increase and as a result, sufficient durability cannot be ensured for long use.
The aforesaid high strength brass should be utilized if importance is given to the improvement of the wear resistance of a contact material, but it is normally hard, having a hardness of Hv 180 or more and therefore presents the drawbacks of poor comformability and a limitation to use under high load, low speed conditions. In addition, high strength brass has significantly high vapor pressure and a high concentration of highly oxidable Zn so that it cannot be bonded to steel by casting. Therefore, high strength brass cannot be utilized in casting-bonding to cylinder blocks, valve plates and the like which are made from iron based material to be used for hydraulic pumps and hydraulic motors, such casting-bonding being one of the chief objects of the invention to be described later.
Regarding wear resistance and seizure resistance, oil-impregnated copper based sintered contact materials have more or less the same problem. Brass based sintered contact materials are also extremely difficult to be sinter bonded to, for instance, iron based materials because of the high Zn concentration of the brass based material, and therefore they cannot be utilized in sinter bonding to cylinder blocks and valve plates.
In recent years, there are strong demands, in view of the environmental problems, towards a ban on use of Pb which is generally contained in lead bronze copper based contact materials.
A prior art bronze based sintered contact material, which has been improved in its characteristics from the above point of view, is disclosed in Japanese Patent Kokai Publication Gazette No. 11-350008 (1999). This publication proposes a double-layered bronze based sintered contact member and its bronze based sintered contact material. In this technique, a powder prepared by blending a bronze powder containing no Pb and 3 to 13 wt % of W powder is dispersed onto a metal backing made of a steel plate; the blended powder and the metal backing undergo sintering and rolling to have high density; and then, sintering is carried out again. According to this technique, since W has good affinity with respect to a bronze matrix and high bonding strength, dropping-off of W due to sliding resistance etc. does not occur. In addition, since W has proper hardness (W: Hv 350 to 500, Mo: Hv 200 to 250), namely, being harder than a bronze matrix and softer than ceramics particles which are too high in hardness and likely to give damage to their mating material, part of the W particles locally protrudes towards the mating contact member, forming an irregular contact surface. The level difference between the convex and concave portions of the irregular contact surface forms a lubricating oil film. Further, since W has a high melting point (3,410° C.), it does not melt unlike Pb. It is considered that, with these features, W keeps good sliding properties free from seizure and non-uniform sliding and does not wear the mating material.
The technique disclosed in the above publication has, however, an economical problem because the W particles to be dispersed for forming a lubricating oil film need to be contained in a large amount (3 to 13 wt %). In addition, as the sliding conditions become harder with increasing circumferential speed and increasing surface pressure, the W particles come into local metallic contact with the mating member, forming agglutinates even though the W particles do not melt unlike Pb. The W particles are not hard enough and therefore a satisfactory function for scratching off the local agglutinates cannot be achieved for stopping the growth of the agglutinates. As a result, a satisfactory improvement in wear resistance cannot be expected and moreover, the generation of a large amount of powder agglutinates leads to a failure in achieving a satisfactory improvement in seizure resistance.
Another technique is disclosed in Japanese Patent Kokai Publication Gazette No. 7-166278 (1995) in which 0.5 to 5 wt % Mo or 0.5 to 15 wt % Fe—Mo is added to a bronze based sintered contact material and/or a lead bronze based sintered contact material, whereby excellent lubrication as well as good affinity with respect to oil are imparted to attain a low friction coefficient and high wear resistance. This technique has proved unsuccessful in that, like Japanese Patent Kokai Publication No. 11-350008, the hardness of the Mo particles is not hard enough, entailing an unsatisfactory improvement in wear resistance and the generation of a large amount of agglutinated powder leads to an unsatisfactory improvement in seizure resistance.
In the producing method described in Japanese Patent Kokai Publication No. 7-166278, bronze based and/or lead bronze based sintered contact materials are compacted to form a green compact which is in turn fit to an iron metal backing plated with copper, and thereafter, a pressure of 10 kg/cm2 or less is applied to carry out pressure sintering and sinter bonding, thereby providing a double layered sintered member having improved mechanical strength in the sintered compact. This producing method involves pressure sinter bonding and therefore imposes many geometric restrictions on its applications as well restrictions on equipment to be employed, which results in not only poor productivity but also high production cost. Additionally, in cases where Pb which is the most effective material for ensuring good comformability is added in a large amount, Pb is likely to flow out of a sintered compact when sintered in a pressurized condition because Pb is a metal element having a low melting point. A large amount of Sn or the like also flows out together with Pb so that not only large amounts of Sn and Pb cannot be contained in the above sintered contact material but also the flow-out of Sn and Pb through the production process causes an environmental problem.
Another well-known method is such that with a view to improving comformability and seizure resistance under severe lubricating conditions with high surface pressure and low sliding speed, a layered solid lubricant such as molybdenum disulfide (MoS2), tungsten disulfide (WS2) or graphite is added to a copper based sintered contact material. This technique has however revealed the problem that since molybdenum disulfide and tungsten disulfide tend to be decomposed into hard copper sulfide (Cu2S) during sintering, it becomes necessary to add these disulfides in large amounts in order to ensure sufficient lubrication. This results in brittleness in the sintered compact and an increase in the cost.
Graphite does not react to a bronze based or lead bronze based sintered material, markedly restrains the sinterability of the sintered compact, weakens the strength of the sintered material, and is hardly wet by Sn rich and Pb rich liquid phases which are generated during sintering. Therefore, addition of graphite presents the problem that sweating becomes significant during sintering, producing a number of melt-off pores. In addition, boundary lubrication is promoted by the facts that as the amount of residual graphite increases, it becomes more difficult to compact the sintered layer and that graphite is a porous substance. In consequence, sliding properties under high-speed oil lubricated conditions cannot be improved as much as expected.
In the field of porous bronze based sintered materials which are used as a friction material for brakes and clutches in applications having utterly different purposes from those of contact materials for bearings, there have been developed materials capable to exhibit a high friction coefficient property for stopping a high speed rotor in a dry, semi-dry or boundary lubricating condition. These materials contain, as shown in Tables 1 to 3 (quoted from the report written by Hanazawa in “Journal of the Japan Society for Composite Materials” 3(1), 8, 1977; “Industries and Products” No. 59; and “Ceramics Data Book 76” p. 336, 1976), large amounts (5 to 15 wt %) of graphite as a base thereby to ensure porosity and low Young's modulus, and further contain heat-resistant metals (e.g., graphite and Mo) which are solid lubricants having excellent heat resistance to prevent the fusion and seizure of the mating material at the time of braking. Further, they contain 3 to 20 wt % hard particles (non-metallic particles) such as SiO2 and mullite thereby restricting the plastic flow of the friction material metal base and properly scraping off the surface of the mating material to achieve an improvement in the wear resistance of the friction material and a stable high friction coefficient.
TABLE 1The compositions of typical metalic cermet friction materials (wt %)ingredientslubricatingmetalic ingredientswear-resistant ingredientsingredientscategoriesCuSnZnsilicamuliteironMographitePbmetalic-167.35.34.47.17.18.8metalic-260˜755˜102˜75˜75˜105˜10metalic-362748712cermet-160520510cermet-25052010510cermet-34735420858
TABLE 2The typical compositions of cermet brake linings for aircraft (wt %)ingredientslubricatingmetalic ingredientswear-resistant ingredientsingredientscategoriesCuSnZnsilicamuliteironMographitePbexample 1Bal.3˜103˜1020˜305˜10example 260520510example 35052010510
TABLE 3The typical compositions of cermet friction materials for general purpose (wt %)ingredientsmetaliclubricatingingredientswear-resistant ingredientsingredientscategoriesCuSnZnsilicamuliteironMographitePbBiexample 4Bal.5˜103˜63˜6 5˜10 5˜10example 5Bal.3˜6 3˜64˜6example 6Bal.5˜103˜53˜510˜1510˜15example 7Bal.3˜6 3˜63˜5 5˜105˜10
If these friction materials are used as a sliding material such as disclosed in the present invention, there arise the following problems.
(1) The exothermic reaction caused at the contact surface by a high friction coefficient becomes a problem.
(2) Since the non-metallic powders are high in hardness, they cause excessive wear of their mating material.
(3) Since a large amount of the non-metallic material is unlikely to be bonded to the metal base, the strength of the sintered compact decreases and the wear resistance of the friction material itself is insufficient. Further, the non-metallic material is likely to drop off the friction surface so that the dropped powder sometimes galls other parts than the friction area.
(4) The friction materials and their mating materials are designed on assumption that they are periodically replaced as consumables.
The suitability of hard dispersion particles as a bronze based sintered friction material is described for example in Published Japanese Translations of PCT international Publication for Patent Applications No. 7-508799 (1995). In this publication, it is disclosed that a material having a friction coefficient as high as possible and independent of temperature, sliding speed, contact pressure and others can be obtained by adding hard particles in an amount of 5 to 40 wt %, the particles having a size of 50 to 300 μm and hardness of HV 600 or more and containing, for instance, carbides of Cr, Mo, W and/or V, nitrides of Al and/or Mo, and/or oxides of Cr, Ni and/or Zr. However, it is apparent that the friction coefficient of this material is so high that it cannot be utilized as a bronze base contact material and this material also suffers from the same problem as described earlier.
Regarding double layered sintered contact members such as engine metals formed by sinter bonding the above lead bronze based sintered contact material to a steel plate, since their production method involves sinter bonding carried out in a condition where an alloy powder having a composition of a lead bronze based sintered contact material is dispersed on a steel plate, the dispersed alloy powder is shrunk during sintering when sinter bonding is performed at a temperature which is at least equal to or higher than the peritectic temperature (about 800° C.) of Cu—Sn, so that the alloy powder is likely to come off during sinter bonding. When sinter bonding a bronze based alloy powder containing no Pb, the sintering temperature needs to exceed the peritectic temperature in order to produce a liquid phase essential for sinter bonding. The dispersed alloy powder shrinks more significantly than the lead bronze based material during sintering and therefore the alloy powder cannot be sinter bonded to the steel plate.
The present invention is directed to overcoming the above-described shortcomings. Therefore, a primary object of the invention is to improve the seizure resistance and wear resistance of a copper based sintered contact material while alleviating attacks on its mating material through scratching-off of local agglutinates on the sliding contact surface, by adding a proper amount of a hard dispersion phase having good agglutination resistance with respect to iron to the copper based sintered contact material, or to provide an inexpensive copper based sintered contact material having an improved critical seizure point by further adding a soft dispersion phase having good agglutination resistance and good lubricity to the copper based sintered contact material to restrain frictional heating caused by the hard phase.
A secondary object of the invention is to provide an inexpensive double-layered sintered contact member which is produced by dispersing a powder of a bronze based or lead bronze based sintered material onto a steel plate and then sinter bonding the dispersed powder to the steel plate, and to which stable sinter-bondability is imparted by adding elements for restraining the sinter shrinkage of the dispersion layer and/or elements for expanding the dispersion layer.